The clearance of antigen-carrying dendritic cells (Ag-carrying DC) by effector CD8+ T cells has been postulated as a mechanism for limiting cytotoxic T lymphocyte (CTL) responses. However, in situations of prolonged Ag exposure, such as chronic infection or cancer, DC elimination by chronically-activated CD8+ T cells may lead to immune suppression. Our preliminary observations indicate that the addition of a tumor-irrelevant (LCMV-gp33) peptide to tumor-lysate loaded DC can increase the efficacy of cancer vaccines administered in therapeutic settings in the presence of pre-existing tumor-specific immunity. Such beneficial effects of rely on the presence of memory-type LCMV-gp33-specific CD8+ T cells;whereas effector-typeLCMV-gp33-specific CD8+ T cells abolished the efficacy of the vaccine. These data suggest that the sequential phases of CD8+ T cell activation are associated with different immunoregulatory activities of CD8+ T cells. Therefore, we propose to delineate the mechanisms underlying the suppressive and helper functions of effector and memory-stage CD8+ T cells, respectively. Additionally, we will examine how these immunoregulatory functions can be, respectively, avoided or exploited for improved cancer immuno-therapies. The work outlined within this proposal will follow three specific aims- (1) To determine the mechanism of immunosuppression by effector CD8+ T cells;(2) to determine the mechanism of helper activity of memory CD8+ T cells;and (3) compare the impact of CD8+ help versus suppression on the survival of vaccine-carrying DC in preventative vs. therapeutic settings. We will use small molecule inhibitors, blocking antibodies and transgenic mice in in vitro assays examining killing of DC by effector-stage CD8+ T cells and the induction of DC maturation and IL-12 secretion by memory-stage CD8+ T cells. Using a DC migration assay in vivo, we will use transgenic mice to determine the pathways involved in helper and suppressor functions in pre-immunized mice carrying memory or effector stage responses, respectively. Lastly, these experiments will also be performed in tumor-bearing mice to delineate the pathways that effect DC-based immunotherapies. Relevance: Therapeutic DC-based vaccines are typically administered in the periphery, where the tumor-specific effector cells predominate and can kill the vaccine. By determining the mechanisms underlying CD8+ T cell immunoregulatory functions, we can design vaccines that are resistant to the suppressive functions and are more capable of receiving the memory cell-derived help, prospectively allowing us to design effective therapeutic vaccines for cancer patients.